[Duugu]

Thank you white cat.
I've tried this:

local button = CreateFrame("CheckButton", "testbutton1", UIParent, "SecureActionButtonTemplate, ActionButtonTemplate")
    button:RegisterForClicks("AnyUp")
    button:SetPoint("CENTER", UIParent, "CENTER")
    button:SetHeight(50)
    button:SetWidth(50)
    button:SetAttribute("type", "spell")
    button:SetAttribute("spell", "1459")
    local button = CreateFrame("CheckButton", "testbutton2", UIParent, "SecureActionButtonTemplate, ActionButtonTemplate")
    button:RegisterForClicks("AnyUp")
    button:SetPoint("CENTER", UIParent, "CENTER", 0, 55)
    button:SetHeight(50)
    button:SetWidth(50)
    button:SetAttribute("type", "equipmentset")
    button:SetAttribute("equipmentset", "1")

The first button does work (at least on my mage *g*).
The second button still does not. :/

I've created a set named "test". GetEquipmentSetIDByName("test") does return '1', so I'm sure that this is the right index.

I've still no idea what's wrong with it or if "equipmentset" is even a valid type. Any other ideas?